Silver paste and aluminum paste are screen printed on the rear surface of a substrate of a solar cell and is fired thereafter. Thus, the aluminum is alloyed with a silicon substrate to form a p+ layer on the rear surface. Because soldering is not possible directly on an aluminum electrode formed of the aluminum paste, a silver electrode is formed of the silver paste on the rear surface of the substrate for taking out the output.
Formed on the rear surface of the solar cell, in a partially overlapping manner, are the aluminum electrode for high output and the silver electrode for taking out the output. This overlapping portion is formed of silicon, silver, and aluminum alloyed together, and is extremely fragile. Therefore, in process such as solder coating, the substrate is easily breakable due to sudden heating and cooling, and also due to stress caused by a difference in thermal expansion coefficients of the soldered substrate and the material of the electrodes, whereby the yield is reduced.
To reduce cost of solar cells, it is necessary to thin substrates that represents a large proportion of the initial cost. However, when substrates are thinned out, solar cells are more likely to break at the fragile part where the three metals are alloyed.
In a conventional solar cell, a plurality of thin lines on the rear surface of the substrate form a silver electrode that takes out the output, and the silver electrode overlaps an aluminum electrode. When the solar cell is heated or cooled suddenly in the process of solder coating or connecting interconnecters, thermal stress is distributed among the thin lines (for example, see Patent Document 1). The electrodes are formed in such a manner that, after paste for a silver electrode is printed on the rear surface of the substrate to form the silver electrode, paste for an aluminum electrode is printed and fired to form the aluminum electrode. Because the silver electrode is fired before printing of the paste for the aluminum electrode, silver and aluminum metals are less likely to react with each other, and formation of a fragile alloy layer of three different metals can be avoided (for example, see Patent Document 1). In another conventional solar cell, quantity of paste applied to form a silver electrode and an aluminum electrode is reduced to avoid formation of an alloy layer of three different metals.
In still another conventional solar cell, a silver electrode includes a protruding portion such that the portion overlaps an aluminum electrode or a portion of it (for example, see Patent Documents 2 and 3).
Patent Document 1: Japanese Patent Application Laid-open No. H10-144943 (p. 3, FIG. 2)
Patent Document 2: Japanese Patent Application Laid-open No. 2004-31740 (pp. 4 and 5, FIG. 1)
Patent Document 3: Japanese Patent Application Laid-open No. 2004-87986 (pp. 4 and 5, FIG. 1)